Significant consolidation and extension of preliminary studies concerning the mechanisms responsible for the tolerant state induced by the injection of semiallogeneic hematopoeitic cells has been achieved in two main areas: (1)\Limit dilution assays have confirmed that the frequency of tolerogen-specific alloproliferative cells, interleukin-2 (IL-2) producers, and cytotoxic T cell precursors (pTcs) are all severely reduced in tolerant mice. Helper cells (alloproliferative, IL-2 producers) are reduced to a level indistinguishable from that of antiself. However, pTcs, although reduced to about 5% of the normal frequency, are clearly detectable although they need exogenous help for their activation. The average avidity of the residual pTcs appears to be lower than that of normal mice, a finding that is analogous to the situation in B cell tolerance to soluble antigen and supports a clonal inactivation hypothesis. (2)\Studies on the effects of anti-I-J antibody administration to tolerant mice have confirmed that antibodies against donor-I-J but not host-I-J cause tolerant mice to reject their skin graft. Furthermore, immunogenetic mapping suggests that the donor-I-J dependent suppression exerts its effects by suppressing responses directed at donor-I-A encoded alloantigens. In vitro studies on anti I-J treated tolerant mice also support this hypothesis since they demonstrate that the anti I-J administration allows tolerant mice to generate help for cytotoxic T cell generation but do not alter the frequency of pTcs. Finally, a series of experiments on the in vitro responsiveness of mice which, although receiving an inoculum of cells as neonates, reject their test skin grafts, has shown (1)\their in vitro responsiveness to be severely impaired; (2)\a positive mixed lymphocyte reaction (MLR) to be indicative of graft injection but a negative MLR not to be indicative of graft acceptance; and (3)\a positive MLR to be required for the generation of cytotoxic T cells. (LB)